DEVELOPMENT OF COLD ROLLED TBF-STEELS WITH A TENSILE STRENGTH OF 1180 MPA

Transcription

1 DEVELOPMENT OF COLD ROLLED TBF-STEELS WITH A TENSILE STRENGTH OF 1180 MPA Florian Winkelhofer, Thomas Hebesberger, Daniel Krizan, Armineh Avakemian and Andreas Pichler, voestalpine Stahl GmbH

2 INHALT 03 Abstract Introduction Experimental Material Dilatometry investigations Annealing simulation Microstructure characterization and mechanical testing Results and discussion Material design Dilatometer results Annealing simulation Mechanical properties of the industrial material Conclusions References

3 3 ABSTRACT Thin sheet steel grades with a bainitic and / or tempered martensitic matrix containing retained austenite belong to the third generation advanced high strength steels. These steel grades, typically named TBF (TRIP-assisted baintic-ferritic), possess in addition to a high tensile strength a very good property combination of high deep drawability and stretch flangeability. Therefore these steel grades are suitable for the manufacturing of complicated safety related parts for the BIW lightweight construction in the automotive industry. Over the last years a lot of activities have been carried out towards development of these grades. Based on this knowledge industrial full scale heats with different chemical composition could be designed. In the present work industrial trials for TBF-grades with a minimum tensile strength of 1180 MPa produced via a conventional continuous annealing line are discussed in detail. Dilatometry was employed to investigate the phase transformation characteristics of the cold rolled materials by isothermal and continuous cooling experiments. In addition annealing simulations were carried out to investigate the microstructure and mechanical properties as a function of overaging temperature. The successfully produced industrial coils with the microstructure consisting predominantly of the bainitic matrix and inclusions of retained austenite exhibit very attractive mechanical properties for this high strength level. Key words: TBF, Q&P, high-ductility steel, retained austenite, industrial material

4 4 01 Introduction The steadily increasing requirements in vehicle safety and fuel economy as well as the challenge to design specific automobile parts in the body in white (BIW) led to the development of advanced high strength steels (AHSS). The first generation AHSS including dual phase (DP), transformation induced plasticity (TRIP), and complex / or multiphase (CP / MP) steels achieved significantly higher strength, in comparison to the conventional high strength steels (HSS), by adding hard phases to the ferritic matrix [1,2]. TRIP steels contain additionally metastable austenite that can transform into martensite during straining resulting in both increased tensile strength and elongation. Further significant improvement of strength and elongation was achieved by development of austenitic steels, which were typically alloyed with high manganese contents of 12-35%. During straining the full austenitic microstructure an twinning induced plasticity (TWIP) effect occurs resulting in a strong work hardening and therefore in an excellent combination of tensile strength and elongation [3]. These steels belong to the second generation AHSS, which were however limited in use due to the high alloying costs. The newest third generation AHSS, including TRIP-assisted bainitic ferrite (TBF) and quench and partitioning (Q&P) steels, exhibit remarkable mechanical properties located between those for the first and the second generation. The high strength of these steels stems from a fine-grained martensitic or bainitic matrix while an increased fraction of retained austenitic inclusions utilize the TRIP effect which leads to enhanced elongations [4,5]. Although in the last years TBF and Q&P steels are object of extensive research work, questions regarding the influence of alloying elements and heat treatment on the microstructure and the mechanical properties still remain. The main challenge in developing TBF or Q&P steels producible on (existing) continuous annealing lines (CAL) or hit dip galvanizing lines (HDG) is to find a suitable alloying and heat treatment combination which leads to the desired properties. In this work three alloying concepts for a TBF steel grade with a minimum tensile strength of 1180 MPa were investigated in detail using dilatometer experiments, annealing simulations and the properties of the annealed full scale material are thoroughly discussed.

5 5 02 Experimental: Material To produce TBF grades on existing CAL an appropriate alloying concept is required. Based on results from recent laboratory investigations [4,5] different alloying concepts were derived for industrial trail heats, which should provide the most preferable mechanical properties and support a cost-effective industrial production. For strength level of 1180 MPa it is known that the carbon content of ~0.2% is required. To obtain a microstructure without or with less polygonal ferrite it is necessary to anneal the material at high temperatures where the microstructure is fully austenitic. The A 3 - temperature is raised as a certain amount of Si is necessary to suppress the carbide formation in the bainite. In order to adjust the A3-temperature to levels below 850 C, required for the industrial production, Mn is added. Furthermore Mn influences the phase transformation, the martensite start temperature as well as the strength in form of solid solution hardening. The chemical composition and the martensite start temperature (MS) of the three investigated alloying concepts are given in table 1. The concept named low_si has a lower Si content of 0.8% compared to the other concepts. Since C stabilizes retained austenite, the heat with lower Si-content contains slightly higher C content. As silicon is known as a ferrite stabilizer, a phase transformation retarding (t.r.) element was additionally added to the concept high_si+t.r. to avoid ferrite formation during the cooling. All three alloying concepts were cast as industrial full scale heats, followed by hot rolling, pickling and cold rolling to a thickness of 1.2 mm. The as-cold rolled samples were taken for the subsequent laboratory investigations. Finally the coils were produced using the CAL. material C [m%] Si [m%] Mn [m%] m.a. t.r. M S [ C] low_si yes no ~350 high_si yes no ~360 high_si + t.r no yes ~360 Table 1: Chemical composition and the martensite start temperature M S of the investigated heats (m.a. micro-alloying element, t.r. transformation-retarding element) Dilatometry investigations In order to obtain a detailed understanding of the phase transformation behavior during cooling from the annealing temperature to the overaging temperature and the isothermal transformation kinetics during overaging temperature, continuous and isothermal heat treatments according to figure 1 were performed. Dilatometer experiments were carried out on a Bähr dilatometer DIL 805 A using as-cold rolled samples with the dimension 1.2 mm x 3.5 mm x 10 mm.

6 6 Figure 1: Sketch of the dilatometer annealing cycles. Left: continuous cooling; right: isothermal transformation The phase fractions at each temperature during the cooling were calculated using the normalized length of the dilatometer sample according to the following equation: = l norm (l - l 0 ) l max 100% where for the same temperature l is the current length whereas l 0 and l max are the lengths under the assumption of 100% austenite and 100% ferrite in the microstructure respectively Annealing simulation To study the influence of the annealing conditions on the microstructure evolution and the resulting mechanical properties, a wide variety of annealing cycles was performed. Cold rolled samples of the heats were heat treated using a Multi-Purpose Annealing Simulator (MULTIPAS) [6] with the time-temperature cycles of the CAL of voestalpine steel in Linz, Austria. The specimens were heated up to the annealing temperature of 850 C followed by soft cooling to the quenching temperature of 720 C and then quenched to overaging temperatures ranging from 475 C to 275 C in 25 C steps as shown in figure 2. Figure 2: Annealing cycles of the continuous annealing line varying the overaging temperature from 475 C to 275 C Microstructure characterization and mechanical testing For the determination of the mechanical properties tensile tests were performed according to the standard EN parallel to the rolling direction. Microstructures were evaluated by optical microscopy using LePera etchant [7]. The amount of retained austenite (RA) that has not been transformed during cooling was determined using the magnetic-volumetric method [8].

7 7 03 Results and discussion: Material design The only way to substantially increase the deep drawability of AHSS steels is to enhance the strain hardening whereby the local necking is shifted to higher elongations. Metastable retained austenite can transform during plastic straining into martensite through the transformation-induced plasticity (TRIP) effect, leading to a desired increase of the strain hardening. In classical TRIP steels the microstructure consists of ferrite matrix with bainitic and retained austenitic inclusions. With such microstructures TRIP steels with strength level up to 800 MPa can be realized. In order to achieve higher strength up to 1180 MPa, a change in matrix design from ferrite to harder phases becomes necessary. To achieve this, a martensitic matrix is used in Q&P steels, while fine grained bainitic matrix can be applied in TBF steels. The difference in hardness between the microstructure constituents is in such microstructure reduced providing an improved stretch-flangeability and bendability. In order to stabilize austenite at the room temperature (RT), Si is commonly added in TBF steels. Si suppresses the carbide formation in the bainite and in the martensite [9-11]. As a consequence the carbon which is rejected from bainite and martensite will enhance the carbon concentration in the austenite stabilizing it at RT Dilatometer results In figure 3 the phase transformations as a function of temperature from the continuous cooling experiments for the cooling rates 10K/s and 40K/s are displayed. Figure 3: Transformed phase fraction during continuous cooling from 850 C down to RT with a) 10K/s and b) 40K/s.

8 8 As shown in figure 3a the phase transformation at a cooling rate of 10K/s started at ~700 C for the material high_si where 10% volume fraction are transformed at ~640 C. In comparison the phase transformation of material low_si started at lower temperature of ~650 C and 10% volume fraction are transformed at ~560 C. A significant different transformation behavior is given for material high_si+t.r. where the transformation starts very slow at ~500 C and reaches 10% volume fraction transformed at the martensite start temperature of ~340 C. At the higher cooling rate of 40K/s a different transformation behavior is shown in figure 3b. The materials low_si and high_si+t.r. displayed no phase transformation down to the martensite start temperature. In comparison to the grade high_si+t.r by the grade high_si the phase transformation started at 690 C, which is typical for the ferrite formation, and reaches almost 20% volume fraction at the martensite start temperature. The cooling rates of 10K/s and 40K/s are close to those from cooling between the annealing and quench temperature (soft cooling) as well as by the subsequent quenching to overaging temperature, as sketched in figure 2. Therefore, it is expected that for the material high_si ferrite will form during the cooling to overaging temperature at the CAL. Figure 4: Isothermal time-temperaturetransformation (TTT) diagram for 10% and 70% transformed phase fraction. In figure 4 the isothermal transformation kinetics for 10% respectively 70% transformation of all three materials is displayed. Comparing the grade high_si and high_si+t.r. the effect of the transformation retarding element is clearly visible in the temperature range between 600 C and 475 C. The transformation did not reach 70% up to 1200 s in this temperature range for the high_si+t.r. grade. In the relevant temperature range for the isothermal phase transformation of TBF grades near the martensite start temperature (below 400 C), the phase transformation kinetics of all grades were alike.

9 Annealing simulation The mechanical properties and amount of retained austenite as a function of overaging temperature are shown in figure 5. The properties of all materials showed the same tendencies with varying the overaging temperature. Tensile and yield strength first decreased with decreasing overaging temperature and then increased again. At overaging temperatures below 325 C, which is below the martensite start temperature, more than 50% of the austenite phase transformed during cooling down to overaging temperature into martensite. Below this temperature the yield strength decreased whereas the tensile strength increased further, thereby the yield ratio decreased which is typical for martensitic steels. The strength-level of the material high_si is about 70 MPa lower than for the other materials. As shown from the dilatometer results in figure 3b the phase transformation occured for material high_si during the cooling to overaging temperature. Consequently, its microstructure includes fractions of ferrite and / or ferritic bainite which resulted in the lower strength. Figure 5: Tensile strength, yield strength, uniform elongation and amount of retained austenite as a function of overaging temperature based on the annealing simulation. The high tensile strength combined with low amount of retained austenite at high overaging temperatures (475 C) is the result of the retarded isothermal phase transformation at these temperatures, as can be seen in figure 4. Therefore the remaining austenite at the end of the overaging time, which is around 400 s, is not sufficiently stabilized with carbon and upon further cooling to room temperature the austenite transforms into fresh martensite. This microstructure consisting of hard martensite and (softer) bainitic ferrite results in a low yield ratio, what is characteristic for DP steels.

10 10 As opposed to tensile strength, uniform elongation and retained austenite showed the maximum values at 425 C. From 350 C to 425 C the elongation increased mainly due to the stabilization of a larger amount of retained austenite and on a smaller scale due to a steadily softer matrix. Above the maximum in elongation at 425 C both the retained austenite and elongation are reduced as a consequence of the T 0 -effect as well as possible of cementite precipitation Mechanical properties of the industrial material Concluded from the laboratory annealing simulations, most suitable properties are to be expected at an overaging temperature of 375 C for the purpose of producing a TBF-steel with a minimum tensile strength of 1180 MPa and high elongation. For the production on the industrial CAL an annealing temperature of 850 C and a overaging temperature of 370 C was used for the materials high_si+t.r. and low_si. Taking into account the dilatometer results and the annealing simulation a higher quench temperature and lower overaging temperature was chosen for the material high_si to strive the achievement of a minimum tensile strength of 1180 MPa. The mechanical properties and the volume fraction of retained austenite for all three materials produced on the industrial CAL are listed in table 2, and the corresponding microstructures are shown in figure 6. material R p0.2 [MPa] R m [MPa] A ue [%] A 80 [%] n 4-6 [-] f [%] low_si high_si high_si + t.r Table 2: Mechanical properties and amount of retained austenite for all three industrial heats produced at the CAL. Figure 6: Microstructure for all three industrial heats produced at the CAL (LePera etching); : ferrite, B : bainite, martensite, R retained austenite.

11 11 Material low_si and high_si+t.r. achieved desired mechanical properties, i.e. R m 1180 MPa and A 80 10%. Due to the higher silicon content of the material high_si+t.r. the fraction of retained austenite is also higher resulting in higher elongations of A 80 13% in comparison to material low_si. Despite the modified annealing parameters for material high_si, certain amount of ferrite transformed in the cooling section of the CAL (figure 6), which led to a tensile strength lower than 1180MPa. The microstructures for all three materials are very fine grained, consisting predominantly of a bainitic matrix and retained austenitic inclusions. In the case of material high_si, some amount of ferrite is additionally present in the microstructure. 04 Conclusions: In this work three alloying concepts for a TBF steel grade with a minimum tensile strength of 1180 MPa and with exceptional elongations for this strength level were thoroughly investigated using the dilatometric experiments and annealing simulations. Furthermore the properties of the annealed full scale material are discussed in detail. Based on the obtained results it can be deduced that for TBF-grades having elongations A 80 13% higher Si contents than 0.8% are necessary. As Si is known as a ferrite stabilizing element some transformation retarding additions are needed to keep the ferrite formation as low as possible during cooling. With respect to the Si level of 0.8% and 1.5%, TBF-grades with R m 1180 MPa and A 80 10% or even A 80 13% can be successfully produced on the industrial CAL. Due to their unique strength / elongation combination these steels are suitable for deep drawing of safety related parts in the BIW light-weight construction. 05 References: [1] De Cooman, B.C.: Curr. Opin. Solid State Mater. Sci., (2004), 8 [2] Speer, J.G., De Moor, E., Findley K., Matlock, D. De Cooman, B. Edmonds, D.: Metall. Mater. Trans., (2011), 42A [3] Bracke, L., Verbeken, K., Kestens, L., Penning, J.: Acta Mater., (2009), 57 [4] Hausmann, K.: PhD theses, TU Munich, (2015) [5] Hausmann, K., Krizan, D., Pichler, A. Werner, E.: Material Science and Technology (MS&T), (2013) [6] Haunschmid, H., Tragl, E., Strutzenberger, H., Angeli,G., Pichler, A., Faderl, J., DeCooman, B.C.: Materials Science and Technology (2009) [7] LePera, F.: Metallography, (1979), 12 [8] Wirthl, E., Pichler, A., Angerer, R., Stiaszny, P., Hauzenberger, K., Titovets, Y.F., Hackl, M.: Proc. of Int. Conf. on TRIP-aided High Strength Ferrous Alloys, (2002) [9] Hofer, C., Leitner, H., Winkelhofer, F., Clemens, H., and Primig, S.: Materials Characterization, (2015), 102 [10] Kozeschnik, E., Bhadeshia, H.K.D.H.: Mater. Sci. Technol., (2008) 24 [11] Hofer, C., Winkelhofer, F., Krammerbauer, J., Clemens, H., and Primig, S.: Materials Today: Proceedings, S925-S928 (2015), 2